This guide is for changing a single member's color, not everyone's!!
1) Admin-> Membergroups-> Create
2) Name it (reccomended that it's named the color of their forum color)
3) Choose a color from
here or any other color code site.
4) Make sure it is a non-post based group.
5) Set permissions to match regular members
6) Profile-> Account settings-> Set primary membergroup to the name of the group.
That should work... If you have any questions please ask me.